1、一、在句中的语法作用不同a．不定式for结构在句中可以作主、宾、表、定、状、同位语：1．It is easy for Tom to do this work．（主语）汤姆做此工作是容易的。2．I'd like for him to come here．（宾语）我喜欢他来这里。3．His idea is for us to travel in two different groups．（表语）他的想法是：我们分成两组旅行。4．Have you heard about the plan for you to go abroad．（定语）你听到让你出国的计划吗？5．The word is too difficult for him to pronounce well．（状语）这单词太难，他念不准。6．In the most schools，it is the custom for the headmaster to declare the newterm start．在大部分学校，校长宣布新学期开始是一个习惯。b．不定式of结构只能在句中作主语。1．It was careless of him to leave his umbrella in the train．他把伞丢在火车上真是太粗心了。2．It is awfully good of you to come to see me off at the station．谢谢你来车站送我。
2、二、表语形容词的语义不同表语形容词的语义不同是区别for结构和of结构的最重要标志。a．在“It is（was）+adj．敫苻匈酃+for sb+to do sth”的句式中，作表语的形容词大多是表事物性质的静态形容词（stative adjective），如：essential，easy，difficult,important，neccessary，（im）possible，hard等等，这类形容词与for后面的名词（代词）关系不密切，无意义上的主表关系。例如：It is neccessary for us to practise speaking English every day．句中不定式逻辑主语“us”与表语形容词“neccessary”无主表关系，我们不能说：We are neccessary to practise speaking English every day．因为“人”（we）不能与表“事物性质的形容词”（neccessary）连用在一起。此句的重点，是强调“每天练习说英语”这件事。同样：It was very difficult for Tom to answer this question．不能说成：Tom was very difficult to answer this question．而可以说：For Tom to answer this question was very difficult．b．在“It is（was）+ adj．+ of sb…”的句式中，作表语的形容词大多是表示人物特征的形容词。如：bold，absurd，brave，careless，careful，clever，considerate，cruel，foolish，good，honest，（un）kind noble，impudent，polite，right，rude，sweet，nutured，silly，stupid，selfish，thoughtful，ungrateful，wicked，unmanly，（un）wise，wrong等等。这些表示人物特征的形容词与of后的名词关系十分密切，有意义上的主表关系。如：1．It is selfish of you to do so．“you”与“selfish”关系密切。“you”既是不定式“to do so”的逻辑主语，又是形容词“selfish”的逻辑主语。此句重点强调的是“人”，（你这人太自私了）故可以说成：You are selfish to do so．又如：2．It is kind of you（= You are kind）to send me a nice present．谢谢你送我这么好的礼物。3．It was stupid of her（=she was stupid）to make such a mistake．出这样的错她真是太蠢了。另外，在这个句式中，“it”可以根据情况改为“that”、“this”，如：4．That is very kind of you．谢谢你。5．This is very good of you to repair the TV set for me．谢谢你帮我修电视机。较长的句子，还可以加逗号，在of sb与to do sth中间停顿一下：6．That was smart of you，to hush up the affair before you heard about it．听到此事之前能保持缄默，你是很聪明的。
3、三、句子的语气有所不同A．在较多的场合中，不定式for结构的句式语气带有情态意义。for sb todo sth．的复合结构，相当于一个有情态动词的主语从句。例如：1．It is neccessary for us to get the preparations done．=（… that weshould get the preparations done．）我们有必要（应该）把准备工作做好。2．It is hard for which one to be given the reward to．=（… which oneshould be given the reward to．）难就难在应该给哪个人授奖。3．In the old society，it was rare for a poor man's son to go to college．（=… that a poor man's son could go to college．）旧社会，穷人的孩子能上大学是很少见的。B．不定式of结构的句式，在语气上往往对前面表人物特征的表语形容词有着某种感叹意味。It is／was adj．of sb相当于How adj of sb it is／was．故在翻译时，多加上“太……”“真是……”“实在是……”等字眼，如前面提到的诸例句：1．It was careless of him to leave his umbrella in the train．（= How carelessof him（it was）to leave his umbrella in the train．）他太粗心了，竟会把伞忘在车上。2．That was stupid of you to make such a mistake．（= How stupid of you tomake such a mistake．）你竟然会犯这样的错误，真是太蠢了。有时，为了加强感叹的程度还可以在表语形容词前加上修饰词：3．That is really kind of you．4．It is awfully good of him to come to see us off at the station．
4、四，由于某些形容词（如：good，right，foolish，wrong，silly…）在语义上具有两重性，既可以表人物的特征，又可以说明事物的性质，因此它们可以使用在for结构中，也可以使用在of结构中，但两种结构在意义上有所区别。试比较：1．It is good for Xiao Wang to travel abroad．（= It's good to travel abroad for-Xiao Wang）中心意思是说“旅行是件好事”，重点在“to travel abroad”不定式所表示的动作。“for Xiao Wang”只表示一种范围。（对于小王来说）2．It is good of Xiao Wang to lend me the money．“of”表示一种所属关系，“good”是“Xiao Wang”的所属特征。中心意思是说“小王心眼真好”，重点在“Xiao Wang”这个人上。又如：3．It is foolish of you to buy that book．（= You are foolish to buy that book．）你这人真傻，竟买了那本书。（强调“你傻”）4．It is foolish for you to buy that book．（=For you to buy that book is fool-ish（= It is foolish to buy that book for you．）你买那本书是愚蠢的。（强调“买那本书”这件事很傻）
